Nanocomposites of hydroxyapatite in polymeric matrices have a number of important applications in bone and cartilage tissue engineering. Most studies to date have focused on systems where the polymeric matrix is a homopolymer gel or block copolymer solution. Use of a block copolymer hydrogel may offer additional control over crystallite size and morphology. Here we report on our efforts to synthesize hydroxyapatite in situ in block copolymer gels. Inorganic ceramic nanocomposites were formed by diffusion of calcium ions into a phosphate-containing Pluronic® F127 hydrogel. Initial pH of the gel prior to nucleation strongly influenced the final crystal structure and morphology. Nucleation at near physiological pH preferentially produced a highly crystalline calcium phosphate phase on the millimeter scale whereas mineral growth at alkaline conditions was found to produce hydroxyapatite crystals in the micrometer size range. Calcium phosphate mineral phases were determined by powder x-ray diffraction (XRD) and substantiated through energy dispersive x-ray spectroscopy (EDS). Morphology of the composites was examined using scanning electron microscopy (SEM). Rheological studies on these systems show that higher elastic moduli are obtained with composites prepared using the in situ synthesis technique as compared to conventionally prepared polymer-hydroxyapatite composites. The biomimetic nature of our investigation suggests that composites formed by this in situ technique my have significant biomaterial and drug delivery applications.